It is great to know that you are so committed to helping your children develop academically. From what you have written it certainly sounds like your son and daughter might be right on track developmentally. Engaging in parallel play and identifying individual interests are both very age appropriate tasks.  Please feel free to speak to your pediatrician or a pre school teacher if you would like more confirmation that they are on target.
 
Rather than promoting the alphabet and letter recognition, I would encourage you to focus on social skill development.  Helping your children to learn how to play nicely side but side and then together will prepare your children to experience greater success in their relationship with one another, with you and with other children and adults they encounter.
 
I know that you would like to teach them more about the alphabet.  If you would like to prompt more letter recognition with your son, I would encourage him to make the sounds that a car might make.  Have him say “beep”, “toot” or “honk”.  Whatever he chooses to say, you could verbally state the name of the first letter of the word your son is saying but it is really too early for anything more involved.  
 
Your daughter is expressing herself through her artwork which is actually a precursor to writing.  Encourage her to talk about what she has created.  Have her tell you a story about her picture.  Verbal skills will only encourage her creative side making it much more likely for her to do well academically.     You can also begin to label some of what she has drawn.  
 
Your son and daughter sound like lovely, talented children.  I would certainly encourage you to let them develop at a rate that allows you to enjoy them every step of the way.  If you begin to feel tired or stressed, there is a very good chance that your children are feeling that way also.  It might be a goal to simply incorporate your academic encouragement as they play.  Do not feel that you need to be doing lessons or making time for “homework” type activities.  
 
In addition, all of the educational experts agree that the very best thing you can do to prepare your children for academic success is to read, read and read to them.  Take time every day to read books and to allow them to read to you.  It doesn’t matter that they are not recognizing actual letters and numbers.  By spending that time with your children and books you will be laying the groundwork for many, many success academic lessons to come.
 
Most importantly enjoy your time with your children.  They grow up way too fast.